The present invention has for its subject matter a device and a process for the pressing of sheets during folding. As it is known, in the production of a book, a booklet or similar items such as, for example, geographical maps and brochures, it is necessary to fold pre-printed sheets of two or more pages over themselves.
To carry out and complete such folding, which must be clear and precise, couples of press rollers are used placed opposite one another with parallel rotation axes. These rollers are of a rather complex construction since they must respond to precise and severe requirements: they must be able to sustain considerable forces theoretically without yielding and present very reduced radial tolerances, in such a way that there are no variations in pressure and tangential velocity along their axes, variations which would give rise to irregular feeding and folding between the various areas of the sheets themselves.
In fact the rollers are subject to heavy mutual approach pressure which is necessary to obtain a neat fold line on the sheets.
The pressure between the two rollers is obtained by means of springs or fluid dynamic cylinders which act on the ends of the rollers, outwith the work area of the same, and by means of restraining reactions again acting correspondingly on the ends of the rollers.
On the contrary, the folded sheets, of various dimensions and thickness, enter centrally between the rollers. It follows that the force of the sheets which tend to push the rollers apart is opposed not where it occurs, but at points distant from each other.
In practice there are inevitably moments when the rollers tend to yield and the pressure exerted by these cannot therefore be uniform transversally to the direction of feeding of the sheets.
Furthermore, the pressing forces in pushing apart the rollers tend to increase since as the elastic elements which determine the approaching of the rollers become more deformed, the more these same pressing forces exerted by them increase.